Some propeller-driven aerial devices such as airplanes, unmanned aerial vehicles (UAVs), and missiles are deployed by launching them into the air from the ground, sea, an airplane, a balloon, or a missile. These devices typically include reciprocating engines, which power the propeller. The reciprocating engines are typically started with a starter, which may be heavy, complex and requires a functioning and charged battery.
Propellers are typically optimized for cruise flight. Accordingly, the aerodynamic configuration of the propeller does not generate large torques when the propeller is not spinning and air is flowing across it at relatively low velocities. Reciprocating engines typically have static friction, sliding friction, and compression resistance, all of which must be overcome before the engine begins to reciprocate. However, a propeller that is optimized to generate sufficient starting torque (e.g., while the air is flowing past it as the vehicle is gliding or falling) will not be efficient during cruise flight or when performing other operations requiring it to propel the vehicle. In one case, the propeller is extracting energy from the air (i.e., serving as a windmill) and in the other case the propeller is adding energy to air as a means of providing a propulsive force for the vehicle.
One approach to addressing the foregoing problem is to outfit the propeller with a variable pitch mechanism that encourages the propeller to windmill. The variable pitch mechanism adjusts the propeller angle of attack relative to the air impacting the propeller. If the variable pitch mechanism has sufficient range of operation, the propeller can be set to create large starting torques during engine start, and then adjusted to provide an efficient propulsive force during cruise and maneuvers. However, the variable pitch mechanism may be heavy, complex, and may reduce reliability. Accordingly, both the battery/starter approach and the variable pitch propeller approach add weight (which is at a premium for operations), cost, complexity, and unreliability to the aerial device.
In still another approach, fixed-pitch propellers can sometimes start an unstarted reciprocating engine if the vehicle dives at a high enough speed during the starting process. However, this method is unreliable and may require very high speeds to enable starting. High dive speeds can increase the structural weight and material strength requirements of the vehicle wings to prevent them from breaking or fluttering or both.